Thermal coating composition

ABSTRACT

An abrasive coating is prepared by plasma spraying a top coating over a bond coating medium. The resultant structure has an improved resistance to corrosion, and a lower thermal conductivity. The coating provides substantially enhanced engine efficiency and improved durability.

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to the following copending U.S. applicationsfiled on even date herewith and commonly assigned to the assignee of thesubject application: U.S. Application Number EH-10096 U.S. Ser. No.08/994,660, filed Dec. 19, 1997 entitled “Thermal Spray Coating Processfor Rotor Blade Tips Using a Rotatable Holding Fixture”, by Zajchowskiet al; U.S. Application Number EH-10095 U.S. Ser. No. 08/994,680, filedDec. 19, 1997, entitled “Tooling Assembly for Positioning Airfoils of aRotary Machine”, by Zajchowski and Diaz; U.S. Application NumberEH-10117 U.S. Ser. No. 08/994,676, filed Dec. 19, 1997, entitled “Shieldand Method for Protecting an Airfoil Surface”, by Zajchowski and Diaz;and U.S. Application Number EH-10118 U.S. Ser. No. 08/994,662, filedDec. 19, 1997, entitled “Method for Applying a Coating to the Tip of aFlow Directing Assembly”, by Zajchowski and Diaz.

TECHNICAL FIELD

This invention relates to thermal spray coating and more particularly,to an improved composition for the coating as applied onto gas turbineengine rotating members.

BACKGROUND ART

Large gas turbine engines are widely used for aircraft propulsion andfor ground based power generation. Such large gas turbine engines are ofthe axial type, and include a compressor section, a combustor section,and a turbine section, with the compressor section normally preceded bya fan section. An annular flow path for working medium gases extendsaxially through the sections of the engine. Each of the fan, compressor,and turbine sections comprises a plurality of disks mounted on a shaft,with a plurality of airfoil shaped blades projecting radially from thedisks. A hollow case surrounds the various engine sections. A pluralityof stationary vanes are located between the disks and project inwardlyfrom the case assembly which surrounds the disks.

During operation of the fan, compressor, and turbine sections, as theworking medium gases are flowed axially, they alternately contact movingblades and the stationary vanes. In the fan and compressor sections, airis compressed and the compressed air is combined with fuel and burned inthe combustion section to provide high pressure, high temperature gases.The working medium gases then flow through the turbine section, whereenergy is extracted by causing the bladed turbine disks to rotate. Aportion of this energy is used to operate the compressor section and thefan section.

Engine efficiency depends to a significant extent upon minimizingleakage of the gas flow to maximize interaction between the gas streamand the moving and stationary airfoils. A major source of inefficiencyis leakage of gas around the tips of the compressor blades, between theblade tips and, the engine case. Accordingly, means to improveefficiency by reduction of leakage are increasingly important. Althougha close tolerance fit may be obtained by fabricating the blade tips andthe engine case to mate to a very close tolerance range, thisfabrication process is extremely costly and time consuming. Further,when the assembly formed by mating the blade tips and the engine case isexposed to a high temperature environment and rotational forces, as whenin use, the coefficients of expansion of the blade tips and the enginecase parts may differ, thus causing the clearance space to eitherincrease or decrease. A significant decrease in clearance results incontact between blades and housing, and friction between the partsgenerates heat causing a significant elevation of temperatures andpossible damage to one or both members. On the other hand, increasedclearance space would permit gas to escape between the compressor bladeand housing, thus decreasing efficiency.

One approach to increase efficiency is to apply an abradable coating ofsuitable material to the interior surface of the compressor housing,which when abraded allows for the creation of a channel between theblade tips and the housing. Leakage between the blade tips and thehousing is limited to airflow in the channel. Various coating techniqueshave been employed to coat the inside diameter of the compressor housingwith an abradable coating that can be worn away by the frictionalcontact of the compressor blade, to provide a close fitting channel inwhich the blade tip may travel. Thus, when subjecting the coatedassembly to a high temperature and stress environment, the blade and thecase may expand or contract without resulting in significant gas leakagebetween the blade tip and the housing.

However, it is critical that the blade tips not degrade when contactedwith the coatings applied to the interior surface of the compressorhousing. To increase the durability of the blade tips which rub againstthe abradable seals, abrasive layers are sometimes applied to the bladetip surface.

The abrasive layers must have a particular combination of properties.They must be resistant to erosion from the high velocity, hightemperature gas streams which at times may carry fine particulate matterwith them. The abradable coating must also be structurally sound toresist the thermal and vibratory strains imposed upon it in use. Inaddition, the intentional contact between the abrasive tip and enginecase creates a demanding, high wear environment for the abrasive bladetip coating.

Considerable effort has gone into the development of abradable coatingshaving the desired combination of properties. For example, Vine et al.,U.S. Pat. No. 4,861,618 discloses a thermal barrier coating which may beused on the airfoil section of a turbine blade. In one embodiment, Vineet al. discloses a NiCoCrAlY bond coat with a ceramic overcoat ofzirconia comprising six to eight weight percent (6 to 8 wt. %) yttria.

This above art notwithstanding, scientists and engineers working underthe direction of Applicant's assignee are seeking to improve thecomposition of the abrasive coating applied to substrates in a gasturbine engine.

DISCLOSURE OF THE INVENTION

According to the present invention, a thermal abrasive coating having animproved resistance to corrosion consisting of from eleven to fourteenweight percent (11 to 14 wt. %) of yttria and the balance essentiallyzirconia.

A primary feature of the coating consisting of 11 to 14 weight percentof yttria is a coating with a lower thermal conductivity as comparedwith prior art coatings containing from six to nine weight percent (6 to9 wt. %) yttria. The thermal conductivity of the coating is one pointone five watts per meter kelvin (1·15 watts/meter-k) as compared withone point four watts per meter kelvin (1·4 watts/meter-k) for coatingscontaining six to nine weight percent (6 to 9 wt. %) yttria.

An advantage of the present invention is the lower substrate temperaturethat results during high temperature frictional heating due to theinteraction between the blade tips and the engine case. The high yttriacontent of the present invention leads to an improved temperaturestability of the coating as compared with coatings containing a lowerweight percent of yttria. Changes in the crystallographic structure ofthe coating are reduced as compared with prior art coatings containinglower weight percent of yttria. Consequently, spalling incidents anddisintegration of the coating resulting from high temperature are alsosubstantially reduced. This, in turn, provides for a coating that iscontiguously bonded on the substrate and thus increases the ability ofthe substrate to resist corrosive effects of the ambient environment.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of the best mode for carrying out the invention andfrom the accompanying drawings which illustrate an embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing the invention process.

FIG. 2 is a partial perspective view, in schematic fashion, showing therelationship of the holding fixture and apparatus for propellingparticles at the tips of an array of rotor blades disposed in theholding fixture, which are used in the present invention.

FIG. 3 is an enlarged view taken along lines 3—3 of FIG. 2 showing therelationship between the plasma spray and the tips of the array of therotor blades.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2 shows a schematic representation of an apparatus for forming andpropelling particles of coating medium and a holding fixture. Aplurality of rotating blades such as compressor blades 10 are positionedin the cylindrical holding fixture 12. The holding fixture has an axisof rotation A_(r). The holding fixture can accommodate a large number ofblades, up to a full stage of blades. The fixture diameter ranges fromabout eighteen to thirty six inches (18 to 36″) (457 to 914 mm),preferably about twenty to twenty-eight inches (20 to 28″) (508 to 711mm) to approximate the size of the flowpath of the engine. The largesize of the fixture can accommodate an entire stage of blades. Selectinga fixture which positions the blades at a radius from the axis ofrotation A_(r) which is the same as the operative radius ensures thelocation of the blade tip approximates closely the radius in the engine.

Each rotor blade has a root and a platform. An airfoil extends from theplatform and terminates in a tip. Each airfoil has a leading edge and atrailing edge. A suction surface and a pressure surface extend betweenthe edges. The blades are oriented such that points on the blade tipsdescribe a circle about the axis of rotation of the holding fixture. Theblade tips face in the outward direction from the holding fixture.

The apparatus for propelling particles toward the blade tips, asrepresented by a spray coating apparatus 14, is in close proximity tothe holding fixture. The spray coating apparatus includes a spray gun 16positioned at the outer diameter of the cylindrical fixture fordepositing the layers. The spray gun is translatable in differentdirections with respect to the holding fixture. The spray coatingapparatus forms a heated plasma including molten particles, such asmolten zirconium oxide particles, which are propelled in the heatedplasma gas stream toward the blades disposed in the fixture.

In one embodiment, the blades are positioned in the holding fixture suchthat adjacent points on the blade tips approximate a surface of rotationsubstantially parallel to the surface of rotation which the blade tipwill experience while in a working engine. As the blades are rotated,the gun moves up and down in a direction substantially parallel to thesurface of rotation of the fixture, coating the blades in sequence.

The thickness of the abrasive coating deposited depends on theapplication of the substrate. In compressor and brush seal applications,the abrasive layer may have a thickness ranging from five to forty mils(5 to 40 mils) (0.13 to 1.02 mm).

FIG. 3 is an enlarged view taken along lines 3—3 of FIG. 2 showing therelationship between the plasma spray propelled from the apparatus forforming and propelling particles and the blade tips disposed in theholding fixture. The circumferential width of the spray can range fromthe size of the circumferential width of the blades to a width ten times(10×) that of the circumferential width of the blades. This enables thespray coating to be deposited uniformly onto the suction and pressuresurfaces of the airfoil of the blade. The phenomenon of overspraying isknown in the art, even in processes that spray coat straight onto bladetips that are stationary. However, the overspray that results from thepresent invention process, coats more airfoil surface area and isapplied uniformly as compared with prior art processes. The oversprayonto the airfoil surfaces provides for better adhesion of the spraycoating onto the blades. The coating is not subject to chipping at theleading and trailing edges as by overspraying and applying the coatingto the leading and trailing edges of the blade and to contiguous areasof the suction and pressure surfaces, as well as to the tip itself, amore durable blade tip may be obtained.

The processing steps of the invention are controlled to produce verticalmicrocracking (essentially perpendicular to the bond coat surface) andare specific to variables such as gun-type and fixture geometry. Thevertical microcracks may extend through a top coating layer to a bondcoating layer. The vertical microcracks do not extend to the substratesurface. The processing steps include the selection of certainparameters. These parameters include rotating the fixture at apreselected speed, angling the gun with respect to the substrate, movingthe gun at a preselected traverse speed, heating the substrate to apreselected temperature, injecting the coating powder at a preselectedrate, and flowing the carrier gas and plasma gases at preselected flowrates. These parameters all influence the structure of the coating andas such should be adjusted to provide uniform coating of compressorblades, or other substrates. In general, it has been found that a closegun-to-substrate spray distance coupled with relatively high spray gunpower results in the desired vertical segmentation or microcracking ofthe coating structure. The parameters described herein were tailored foruse with an F-4 model air plasma spray gun purchased from PlasmaTechnics, Inc., now supplied by Sulzer Metco having facilities inWestbury, N.Y., and various diameter cylindrical fixtures depending onsubstrate configuration. As will be realized, the parameters may varywith the use of a different spray gun and/or fixture. Accordingly, theparameters set forth herein may be used as a guide for selecting othersuitable parameters for different operating conditions.

The process for controllably applying spray coating as flow charted inFIG. 1, includes a number of interrelated steps beginning with providingblades having clean, exposed blade tips and protected airfoil and rootsurfaces typically provided by masking. Conventional cleaning andpreparation of the blade tip prior to application of the abrasive layershould be conducted. In the practice of the present invention, forexample with a blade tip as shown in the figures, the surface of theblade tip is cleaned and roughened to enhance adherence of subsequentlyapplied coating materials. Such cleaning can include mechanical abrasionsuch as through a vapor or air blast type process employing dry orliquid carried abrasive particles impacting the surface.

Prior to cleaning the surface, blades are suitably masked as shown inU.S. Application Number EH-10117 U.S. Ser. No. 08/994,626, entitled“Shield and Method for Protecting an Airfoil Surface”, by Zajchowski andDiaz, herein incorporated by reference.

The process includes propelling a spray of particles of softened bondcoating medium toward the blade tips. The step of propelling the coatingmedium includes the step of forming a spray of particles of softenedbond coating medium in the spray coating apparatus. This step includesflowing bond coat powder and carrier gases into a high-temperatureplasma gas stream. In the plasma gas stream, the powder particles aremelted and accelerated toward the substrate. Generally, the powder feedrate should be adjusted to provide adequate consistency and amount ofbond coating. The bond coat powder feed rate ranges from thirty tofifty-five grams per minute (30 to 55 grams/min). Carrier gas flow(argon gas) is used to maintain the powder under pressure and facilitatepowder feed. The carrier gas flow rate ranges from four to eightstandard cubic feet per hour (4 to 8 scfh) (1.9 to 3.8 standard litersper minute (SLM)). Standard conditions are herein defined as about roomtemperature (77° F.) and about one atmosphere of pressure (760 mmHg)(101 kPa).

The gases that make up the plasma gas stream comprise of a primary gas(argon gas) and a secondary gas (hydrogen gas). Helium gas may also beused as a secondary gas. The primary gas flow rate in the gun rangesfrom seventy-five to one hundred and fifteen standard cubic feet perhour (75 to 115 scfh) (35 to 54 SLM), while the secondary gas flow rateranges from ten to twenty-five standard cubic feet per hour (10 to 25scfh) (4.7 to 12 SLM). Spray gun power generally ranges from thirty tofifty kilowatts (30 to 50 KW).

The process then includes the step of translating the spray of softenedbond coating medium at a distance ranging between about four to sixinches (4 to 6″) (102 to 152 mm) from the blade tips, between a firstand second position. In one embodiment, the spray gun is moved in adirection substantially parallel to the surface of rotation of theholding fixture. Spray gun traverse speed during bond coat depositionranges from six to twelve inches per minute (6 to 12 in/min) (152 to 305mm/min).

Further, the process includes passing the blades through the spray ofparticles of softened bond coating medium by rotating the fixture aboutits axis of rotation. This step includes heating the blades to atemperature of two hundred to four hundred and fifty degrees Fahrenheit(200 to 450° F.) by passing the blades in front of the spray gun and hotplasma gas stream. The step of passing the blades through the spray ofparticles of softened bond coating medium also includes cooling theblades and the coating layer deposited by rotating them away from thespray gun. Additional cooling of the blades can be provided by directinga cooling air stream or cooling jet on the blades or the fixture.Independent sources of heating can also be provided to heat the bladesprior to the blades entering the spray of particles of coating medium.The independent heating source would allow for control of bladetemperature without adjusting the spray gun to provide heating.Specifically, during bond coat deposition, the cylindrical fixturerotates at a speed which ranges from twenty to seventy-five revolutionsper minute (20 to 75 rpm), depending on substrate diameter. The surfacespeed of the blades ranges typically from one hundred and twenty-five tothree hundred surface feet per minute (125 to 300 sfpm).

The coating process then includes the step of forming a spray ofparticles of softened top coating medium. This step includes flowing topcoat powder and carrier gases into the high-temperature plasma gasstream. Generally, the powder feed rate should be adjusted to provideadequate mix to cover the substrate, yet not be so great as to reducemelting and crack formation. Top coat powder feed rate ranges fromfifteen to forty grams per minute (15 to 40 grams/min). Carrier gas flow(argon gas) is used to maintain the powder under pressure and facilitatepowder feed. The flow rate ranges from four to eight standard cubic feetper hour (4 to 8 scfh) (1.9 to 3.8 SLM). As described hereinabove,standard conditions are herein defined as about room temperature (77°F.) and about one atmosphere of pressure (760 mmHg) (101 kPa).

The step of forming a spray of particles of softened top coating mediumincludes the injection of the top coat powder angled such that itimparts a component of velocity to the powder which is opposite to thedirection of flow of the plasma toward the rotating fixture. Theprojection of the injection angle in a plane perpendicular to the axisof rotation of the holding fixture lies in a range from sixty-five toeighty-five degrees (65 to 85°). This injection angle serves tointroduce the top coat powder further back into the plasma plume, thusincreasing the residence time of the powder in the plasma gas stream.The increased residence time in the plasma gas stream provides forbetter melting of the powder particles.

Primary gas flow (argon gas) in the gun ranges from fifty to ninetystandard cubic feet per hour (50 to 90 scfh) (24 to 43 SLM). Similarly,secondary gas flow (hydrogen gas) in the gun ranges from ten to thirtyscfh (10 to 30 scfh) (4.7 to 14 SLM). Spray gun power generally rangesfrom thirty to fifty kilowatts (30 to 50 KW).

The process further includes the step of translating a spray of softenedtop coating medium at a distance ranging from three to four inches (3 to4″) (76 to 102 mm) from the blade tips, between a first and secondposition in a direction substantially normal to the plane of rotation ofthe holding fixture. Spray gun traverse speed across each part duringdeposition ranges from two to ten inches per minute (2 to 10 in/min)(50.8 to 254 mm/min). The gun-to-substrate distance may be varied withthe intent of maintaining the appropriate temperature level at thesubstrate surface. A close gun-to-substrate distance is necessary forsatisfactory vertical microcracking.

The process further includes the step of passing blades through thespray of particles of softened top coating medium by rotating thefixture about its axis of rotation, wherein the step includes heatingthe blades by passing the blades in front of the spray gun. Thetemperature of top coat application is the temperature measured at thesubstrate at the time of applying the top coating. The temperature ofapplication may vary from three hundred to eight hundred and fiftydegrees Fahrenheit (300° F. to 850° F.). The actual temperature ofapplication is preferably maintained at a relatively constant levelvarying from about ±five to ten percent (±5% to 10%) of a predeterminedtemperature, depending upon the size of engine element coated, and thesubstrate on which the top coating is sprayed.

The step of passing the blades through the spray of softened particlesincludes the step of cooling the blades. Additionally, external coolingmay be used to control deposition temperature.

This process results in layers of bond and top coating beingsequentially deposited onto the blade tips in a surface of rotationsubstantially parallel to the surface of rotation which the bladesdescribe when rotating in operating conditions. While the phenomenon isnot well understood, it is believed that by depositing coating layersone at a time in an orientation substantially parallel to the surface ofrotation that the coating layers will experience in an operating engine,the process confers an advantage as it provides relatively uniformmicrocracking of the coating in a radial direction. This results inrelatively uniform stresses in the coating structure during operativeconditions.

The bond coating medium provides an oxidation resistant coating.Typically, the bond coating material is a nickel-aluminum alloy.However, the bond coating medium may alternatively comprise of MCrAlY orother oxidation resistive material.

The top coating medium used consists essentially of from eleven tofourteen weight percent (11 to 14 wt. %) of yttria and the balanceessentially being zirconia. This top coating composition with a highyttria content provides improved resistance to corrosion, as well asbetter temperature stability of the top coating ceramic material. Theimproved stability of the top coating material decreases the likelihoodof spalling of the material. Thus, the substrate material remainsprotected from the corrosive effects of the sulfides and salts from theambient environmental conditions.

Further, the high yttria content of the top coating material providesfor a material having a lower thermal conductivity as compared withmaterial prepared with lower yttria content. The thermal conductivityfor the eleven to fourteen weight percent (11 to 14 wt. %) yttria isapproximately one point one five watts per meter Kelvin (1.15watts/meter-k) as compared to a thermal conductivity of one point fourwatts per meter Kelvin (1.4 watts/meter-k) for a coating consisting ofseven to nine weight percent (7 to 9 wt. %) of yttria. The lower thermalconductivity of the coating provides an advantage during rub events inan operational engine when the blade tips make contact with the innersurface of the engine case. The rub generates a step input of frictionalheat in the contacting surfaces. This heat has to be removed. The lowerthermal conductivity of the blade tip coating, comprising eleven tofourteen weight percent yttria, provides for heat transfer from theblade tips via convection and radiation. The process of conduction isnot used for heat removal. Thus, it is believed that lower thermalconductivity of the coating would result in a lower substratetemperature as the coating does not conduct heat down to the bond coatand therefore to the substrate as compared with substrates coated withcompositions containing a lower weight percent of yttria. The propertiesof the base metal substrate, thus are unaffected by heat as in the caseof compressor blade tips, and thus retains the coating better inservice.

A primary advantage of the present invention is the quality of coatingapplied to the tips of rotor blades which results from using the processto distribute among a multiplicity of the rotor blades any variations inthe process flow parameters affecting the stream of particles propelledagainst the tips. Due to the rotating fixture, a number of blades passthrough the spray of softened coating medium. Any variations in the flowparameters such as variations in spray intensity, temperature,composition and feed of powders to the spray are distributed over anumber of blades that pass through the spray during the period ofvariation. This ensures that one rotor blade tip does not receive all ofthe variations in coating. As a result, the coating process of thepresent invention provides for a more uniform coating and has lesssensitivity to process variations than a process using a stationaryfixture in which all variations are deposited only on a single blade.Further, the coating is applied in layers that are approximatelyparallel to the location of that part of the tip of the rotor bladeabout the axis. By selecting a fixture which positions the tips at aradius from the axis of rotation A_(r) which is the same as theoperative radius ensures the location of the tip approximates closelythe radius in the engine. As a result, the coating is substantiallyparallel to the axis of rotation of the fixture and the coating layerfollows approximately the surface of rotation which the coating layerwill experience during operation of the engine. It is believed theorientation of the coating will enhance performance of the coating.

Another advantage is the reproducible and reliable process that resultsdue to the use of the control parameters. This process can be used torepetitively apply bond coating onto substrate surfaces or top coatingonto bond coating layers.

Another advantage is the ease and speed of application of the coating onthe surfaces of a large number of blades at a given time which resultsfrom the size of the holding fixture and process which accommodates amultiplicity of blades. Using a holding fixture that accommodates anumber of blades, the resultant fixturing time is minimized. In certainembodiments, an entire stage of blades can be coated.

Another advantage of the present invention is the application of coatingto substrates without the use of additional heating apparatus for thesubstrates. During coating deposition, the optimum amount of heatrequired is transmitted to the substrates through the plasma gas and themolten coating powder. The rotor blade is not overheated during thecoating process. As a result, a rotor blade can be coated withoutchanging the substrate microstructure or properties.

In the following examples, the best mode practices, just described, aregenerally followed. An F-4 model air spray gun purchased from PlasmaTechnics, Inc., now supplied by Sulzer Metco, having facilities inWestbury, N.Y., is used for all the following examples.

EXAMPLE I

In this practice of the invention, small nickel rotor blades arepositioned in a holding fixture measuring twenty-four inches (24″) (610mm) in diameter.

For the bond coat application, the spray gun is powered to aboutthirty-five kilowatts (35 KW). The bond coat powder feed rate isforty-five grams per minute (45 gms/min). The primary gas (argon) flowrate is ninety-five scfh (95 scfh) (45 SLM) and secondary gas (hydrogen)flow rate is eighteen scfh (18 scfh) (8.5 SLM). The spray gun ispositioned five and one-half inches (5.5″) (140 mm) away from the bladetip surfaces. The holding fixture rotation speed is forty revolutionsper minute (40 rpm) while the spray gun traverse rate is nine inches perminute (9″/min) (229 mm/min).

For the top coat application, the plasma spray gun is powered to aboutforty-four kilowatts (44 KW). The top coat powder feed rate istwenty-two grams per minute (22 gms/min). The primary gas (argon) flowrate is sixty-seven scfh (67 scfh) (32 SLM) and secondary gas (hydrogen)flow rate is twenty-four scfh (24 scfh) (11 SLM). The spray gun ispositioned three and one-quarter inches (3.25″) (83 mm) away from theblade tip surfaces. The holding fixture rotation speed is thirtyrevolutions per minute (30 rpm), while the spray gun traverse rate issix inches per minute (6″/min) (152 mm/min). The blade temperatureduring top coat application is six hundred plus/minus twenty-fivedegrees Fahrenheit (600±25° F).

The bond coat composition is ninety-five weight percent nickel (95 wt.%) and five weight percent aluminum (5 wt. %). This composition resultsin an adherent bond coat on the blade tips.

The top coat composition is twelve weight percent yttria (12 wt. %) andthe balance essentially being zirconia. The process and the compositionof the coatings results in a desired splat structure having verticalmicrocracks being deposited on the blade tips. The vertical microcracksextend through the top coating layer to the bond coating layer.

EXAMPLE II

In this practice of the invention, titanium rotor blades, twice the sizeof the blades used in Example I, are positioned in a holding fixturemeasuring twenty-four inches (24″) (610 mm) in diameter.

For the bond coat application, the spray gun is powered to aboutthirty-four kilowatts (34 KW). The bond coat powder feed rate isforty-five grams per minute (45 gms/min). The primary gas (argon) flowrate is ninety-five scfh (95 scfh) (45 SLM) and secondary gas (hydrogen)flow rate is eighteen scfh (18 scfh) (8.5 SLM). The spray gun ispositioned five and one-half inches (5.5″) (140 mm) away from the bladetip surfaces. The holding fixture rotation speed is forty revolutionsper minute (40 rpm), while the spray gun traverse rate is nine inchesper minute (9″/min) (229 mm/min).

For the top coat application, the plasma spray gun is powered to aboutforty-four kilowatts (44 KW). The top coat powder feed rate istwenty-two grams per minute (22 gms/min). The primary gas (argon) flowrate is sixty-seven scfh (67 scfh)(32 SLM) and secondary gas (hydrogen)flow rate is twenty-four scfh (24 scfh) (11 SLM). The spray gun ispositioned three and one-quarter inches (3.25″) (83 mm) away from theblade tip surfaces. The holding fixture rotation speed is thirtyrevolutions per minute (30 rpm), while the spray gun traverse rate issix inches per minute (6″/min) (152 mm/min). The blade temperatureduring top coat application is four hundred and twenty-five plus/minustwenty-five degrees Fahrenheit (425±25° F).

The bond coat composition is ninety-five weight percent nickel (95 wt.%) and five weight percent aluminum (5 wt. %). This composition resultsin an adherent bond coat on the blade tips.

The top coat composition is twelve weight percent yttria (12 wt. %) andthe balance essentially being zirconia. The process and the compositionof the coatings results in a desired splat structure having verticalmicrocracks being deposited on the blade tips. The vertical microcracksextend through the top coating layer to the bond coating layer.

EXAMPLE III

In this practice of the invention, large titanium rotor blades, threetimes the size of the blades used in Example I, are positioned in aholding fixture measuring thirty-four inches (34″) (864 mm) in diameter.

For the bond coat application, the spray gun is powered to aboutthirty-five kilowatts (35 KW). The bond coat powder feed rate isforty-five grams per minute (45 gms/min). The primary gas (argon) flowrate is ninety-five scfh (95 scfh) (45 SLM) and secondary gas (hydrogen)flow rate is eighteen scfh (18 scfh) (8.5 SLM). The spray gun ispositioned five and one-half inches (5.5″) (140 mm) away from the bladetip surfaces. The holding fixture rotation speed is thirty-tworevolutions per minute (32 rpm), while the spray gun traverse rate isnine inches per minute (9″/min) (229 mm/min).

For the top coat application, the plasma spray gun is powered to aboutforty-four kilowatts (44 KW). The top coat powder feed rate istwenty-two grams per minute (22 gms/min). The primary gas (argon) flowrate is sixty-seven scfh (67 scfh) (32 SLM) and secondary gas (hydrogen)flow rate is twenty-four scfh (24 scfh) (11 SLM). The spray gun ispositioned three and one-quarter inches (3.25″) (83 mm) away from theblade tip surfaces. The holding fixture rotation speed is twenty-tworevolutions per minute (22 rpm), while the spray gun traverse rate istwo inches per minute (2″/min) (51 mm/min). The blade temperature duringtop coat application is three hundred and twenty-five plus/minustwenty-five degrees Fahrenheit (325±25° F).

The bond coat composition is ninety-five weight percent nickel (95 wt.%) and five weight percent aluminum (5 wt. %). This composition resultsin an adherent bond coat on the blade tips.

The top coat composition is twelve weight percent yttria (12 wt. %) andthe balance essentially being zirconia. The process and the compositionof the coatings results in a desired splat structure having verticalmicrocracks being deposited on the blade tips. The vertical microcracksextend through the top coating layer to the bond coating layer.

Although the invention has been shown and described with respect todetailed embodiments thereof, it should be understood by those skilledin the art that various changes in form and detail thereof may be madewithout departing from the spirit and the scope of the claimedinvention.

What is claimed is:
 1. A coating system for compressor blade tips in agas turbine engine which comprises: a. a metallic substrate forming atleast part of a compressor blade tip; b. an adherent bond coat on saidsubstrate, said bond coat exhibiting a first abradabilitycharacteristic; c. a top coat of eleven to fourteen weight percent (11to 14 wt. %) yttria and the balance essentially zirconia, said top coatexhibiting a second abradability characteristic; wherein the secondabradability characteristic of the top coat is higher than the firstabradability characteristic of the bond coat and said coating systemincludes microcracks essentially perpendicular to the bond coat whichextend through the top coat to the bond coat.
 2. A gas turbine engineseal system comprising a compressor component, and a cooperating sealmember, the compressor component having an abradable coating disposedthereon wherein said compressor component includes, a compressor bladehaving a tip having an abrasive thermal top coating on at least aportion of said tip, the abrasive thermal top coating having an improvedresistance to corrosion, said coating consisting of from eleven tofourteen weight percent (11 to 14 wt. %) yttria and the balanceessentially zirconia said compressor blade tip located and adapted formotion, relative to said seal member, so that said abrasive thermal topcoating can interact with abradable seal member to provide sealing. 3.The abrasive thermal top coating of claim 2, wherein the coating has athermal conductivity not exceeding one point one five watts per meterkelvin (1.15 watts/meter-k) which results in lower substratetemperatures as compared with coatings having a lower weight percent ofyttria.